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12/07/2018, 4:21 am EST

Global Soil Moisture Outlook

Past 3 months: Much wetter U.S., strong Europe drought. Next 3 months: Great Plains is drier. Fig. 1: November 2018 global soil moisture % of normal and 3-month trend. Observation discussion: Large regions of significant soil moisture change over the past 3 months were evenly split across the globe with both 5 large areas of regional wetter or drier regime adjustments (Fig. 1). Beginning in North America a vast area of much wetter than normal soil moisture has developed across Mexico, the central and eastern U.S. and far southeast portions of Canada. The wetter trend is associated with influence of warm SSTA in the southeast Pacific and off the East Coast of the U.S. providing above normal low atmospheric moisture entrained into the upper flow south of a stronger than normal polar vortex in eastern Canada. In west/northwest Canada an upper ridge pattern has persisted northeast of a warming northeast North Pacific. Beneath the ridge pattern Western Canada drought has intensified. In South America a wet trend centered on Paraguay is also affecting northern Argentina and southwest to central Brazil the past 3 months. Most recent wetter weather has been caused by a stronger than normal polar vortex over or in-between Antarctica and South America. The polar vortex has ejected short wave energy across South America bringing stronger than normal cold fronts into the early summer season some of which have deposited excessive rain in Argentina while stalling once shifting north over Brazil. Interestingly, northeast Brazil has turned wetter in the current pattern. The mega-drought affecting Europe is mostly related to an issue present nearly all of 2018. The polar vortex across eastern Canada to Greenland and sometimes the northern North Atlantic has been stronger than normal most of this year causing an upper ridge pattern to emerge over Europe and stagnate. The upper ridge has caused persistent dryness and following heat during the summer season which further enhanced drought. The dryness has strengthened during autumn from Central Europe eastward and to the east of the Caspian Sea. Upper level energy ejected from the persistent upper trough over Eastern Canada to Greenland during the past 3-6 months has turned southeast and caused a wet pattern to emerge in Northwest Africa. The lack of tropical convection in the equatorial portion of Africa is also related to dryness in South Africa where drought is intensifying. A combination of an emerging positive phase of the Indian Ocean Dipole (+IOD) and general El Nino condition in the tropical Pacific has been the catalyst for a somewhat sharp reversal to drier soil moisture conditions across Southeast Asia and India. However, an active Madden Julian oscillation (MJO) has favored a wetter climate in parts of Indonesia while stronger than normal cold fronts into Australia into the early warm season have caused the strong drought to weaken slightly. Fig. 2: November 2018 global soil moisture % of 30-year normal and 3-month forecast. Forecast discussion: A weak-to-moderate El Nino is forecast the next 3 months but due to lack of intensity of warm ENSO regional climate influences gain relevance on the global 3-month soil moisture outlook (Fig. 2). In North America the weak-to-moderate El Nino favors the wet soil moisture trend in Texas and Mexico continuing and likely spread across most of the southern U.S. including much of the East U.S. There are questions on how wet the winter climate will be in California which is usually quite wet during an El Nino. To the north of the southern U.S. storm track much of the Great Plains should trend drier, possibly sharply drier. Excessive precipitation is not expected but the projected position of the polar vortex coupled with occasional entrainment of above normal low atmospheric moisture across the warmer than normal northeast Pacific Ocean should produce beneficial precipitation to ease Western Canada drought. In South America there is no major change expected to the current soil moisture conditions. Subtropical ridging can bring periods of dryness to both Brazil and Argentina. However, the subtropical ridge is not expected to be particularly strong and in its absence periods of wet weather can occur. A changeable climate pattern implies no drought concerns. However, there is a drought concern in South Africa where already established dryness is expected to expand during summer as attendant anomalous heat acts as a feedback mechanism to intensify the drought. Further north across Europe the intense drought condition may ease slightly but only northeast Europe/western Russia receives above normal precipitation due to a wet fetch off the open waters of the North Atlantic to the north. The extended-range forecast well into 2019 anticipates the Europe drought expanding eastward just prior to the 2019 northern hemisphere warm season! Heavy snows are expected during winter to supply moisture to ease the northeastern Russia drought. However, the effects of weak-to-moderate El Nino favor dryness for India, Indonesia and expansion of Australian drought westward. There may be heavy rains in Eastern Australia associated with tropical convection easing the East Australia drought.